Ars Technica writes about a recent federal appeals court ruling:
A federal appeals court has overruled a lower court ruling that, if sustained, would have severely hampered the enforceability of free software licenses.
The Federal Circuit appears to have been heavily influenced by the Stanford brief, as it specifically cited Creative Commons, MIT, Wikipedia, and various free software projects as examples of organizations that benefit from copyleft licenses. In a short, clearly-reasoned opinion, the Federal Circuit summarized the public benefits of public licensing and found that the district court had dismissed its terms too lightly. Unlike the lower court, the appeals court seemed to understand that reciprocity lay at the heart of free software licenses. Just as traditional software firms thrive on the exchange of code for money, free software projects thrive on the exchange of code for code. The Federal Circuit recognized that "there are substantial benefits, including economic benefits, to the creation and distribution of copyrighted works under public licenses that range far beyond traditional license royalties." Allowing those rules to be flaunted undermines the free software model.
The main issue of the case was whether violating the copyleft license was a breach of contract or a copyright infringement. Now it's clear that it's the latter which strengthens the enforceability of such licenses considerably.